The invention relates to an optical fiber cable having filler rods that are strategically placed to protect the optical fibers located in buffer tubes.
As smaller and higher fiber density fiber optic cables have become emphasized, overall cable strength and robustness have been sacrificed. Outer sheath thicknesses have been reduced, strength elements have been reduced, and buffer tube sizes have been reduced, all in an effort to reduce both the cost and size of fiber optic cables. In order to continue to meet standard industry requirements as well customer expectations in terms of strength and robustness, yet have a smaller cable, a novel approach is needed.
The invention is particularly applicable to stranded loose tube cables in which buffer tubes are stranded around a central strength member. In these types of cables, it is very common to use filler elements, also known as filler rods, in place of buffer tubes, when buffer tubes are not necessary for a particular design.
FIG. 1 shows a conventional stranded loose tube cable 8 having a central strength member 10, buffer tubes 12 housing optical fibers 14, and a sheath 16. The cable has six positions for receiving buffer tubes 12, each having twelve optical fibers 14 for a total fiber count of 72. FIG. 2(a) shows a related optical fiber cable 8 with four buffer tubes 12 and two filler elements 18 that fill the void of omitted buffer tubes for a total fiber count of 48. Traditionally, the filler elements 18 are placed in the last two positions, adjacent each other as shown in FIG. 2(a). As a result, the filler elements 18 are diametrically opposite the buffer tubes 12. Also, U.S. Patent Publication No. 2004/0071416 discloses a cable having three filler elements that are diametrically opposite respective buffer tubes.
FIG. 2(b) is a schematic view of the FIG. 2(a) cable 8 showing the effect of a crush load applied to the cable. The filler elements 18 are typically more rigid than the buffer tubes 12. The relatively soft buffer tubes 12 are more likely to deform than rigid filler elements 18, whereas the more rigid filler elements 18 are more likely to resist load. As shown in FIG. 2(b), when the filler elements 18 are placed in the last two positions, adjacent to each other, the load will travel through the most rigid members first. In this case, load path will be through the buffer tubes 12 and filler elements 18. The buffer tubes 12 opposite the filler elements 18 will support the load, and deform if the load is greater than the strength of the buffer tubes 12. In the cable of FIG. 2(b), the buffer tubes 12 opposite the filler elements 18 will be deformed more than the other buffer tubes that oppose each other thus jeopardizing the optical fibers in the buffer tube.